The present invention relates to simulation on a lab workbench of conditions that would be encountered by a mobile device during a so-called drive test, which involves transporting the mobile device along a course so that it encounters fading and changing wireless access points, which normally are used to connect the mobile device to a wireless network, but in this case are used to locate the device. The instrument and method also support parametric testing of transceivers used for WiFi positioning and, optionally, coordination between GNSS positioning and WiFi positioning.
Global Navigation Satellite Systems (“GNSS”) provides excellent positional accuracy in a wide range of environments. A GNSS receiver determines the time a signal from a GNSS satellite takes to travel to the receiver and converts this time to distance using the constant speed of electro-magnetic waves. By calculating the distance to three, or more, satellites and knowing the position of each satellite, the receiver can compute its exact position on the Earth's surface. The ubiquitous GPS system used in the US is one variety of GNSS.
GNSS struggles where signals can be obscured, such as in the “urban canyon” and particularly indoors. In such circumstances, alternative positioning methods are useful to support commercial location-based services and emergency location schemes.
Other positioning systems determine approximate locations from cellular tower signals, instead of satellite signals. In the US, South Korea and Japan where CDMA mobile phone systems are deployed, the system supports a pseudo-satellite capability called AFLT. This uses the time-synchronous nature of the signals; phones can measure the transmission delay from the cell tower. Using knowledge of the cell tower position, the network can determine a phone's location within 100 m or so. For other types of mobile phone systems, popular mobile phone-based positioning technologies include cell-ID and enhanced cell-ID. With cell-ID, receiving a signal from a particular cell tower suggests a location within a km or less. With enhanced cell-ID, sectored cells can give an angle of arrival to narrow down a user's location.
Widespread use of WiFi for wireless access points presents a new source of location signals. Access points (“APs”) are widely visible to mobile devices, both commercially deployed as ‘Hot-Spot’ APs in cafés, bars, shopping centers, railway stations, etc., and privately hosted APs in homes and businesses. The approximate range to an AP can be determined by measuring the power level of the WiFi beacon transmission, since signal power decreases approximately with the square of the distance. The IEEE 802.11 standards for WiFi include a beacon frame as a type of management frame. The typical beacon frame is about fifty bytes long and includes source identification information. The destination address is set to a constant, such as all ones, so that all receivers within range will process the beacon.
Typically, software within the mobile device sends details of the visible APs, from their beacons, to positioning service servers together with identifications of nearby cellular phone base stations. The location server accesses a continuously updated database of APs. The location server calculates and returns the location to that device. Alternatively, a location service provider can provide a power-level or visibility map for APs in an area. The mobile device uses the AP location data to determine its location based on the signature of power levels, applies a form of maximum likelihood algorithm, or from a technique similar to fingerprinting.
There are several suppliers of WiFi positioning services that typically use data generated by driving surveys and/or subscriber reports, or through direct on-line registration of deployed access points with the supplier. It is believed that estimates of the location and transmission power of unregistered access points are made using multiple observations of power of basic signaling at known locations. Suppliers can offer a tiered accuracy service which starts with GPS, drops to WiFi, used when GPS coverage is poor, and finally degrades to cell-ID.
An opportunity arises to perform device testing that does not rely on field testing, but instead emulates beacon signals from multiple WiFi Access Points. Better, more easily configurable and controllable, more repeatable testing and development of positioning systems may result.